Origin of the Iron Bands in Supernovae Spectra

Abstract

THE visible-region crystal-field spectra of Fe²⁺ and Fe³⁺ ions located in silicate minerals show several similar features, in particular a number of relatively sharp bands marking field-independent transitions, which are difficult to assign unless attendant chemical analyses are available. Three absorption bands at 6200 Å, 4430 Å and 3800 Å in optical spectra of Type I supernova were recently assigned^1,2 to transitions within octahedral Fe³⁺ ions in andradite garnet, while two other bands at 5100 Å (19,600 cm⁻¹) and 5700 Å (17,500 cm⁻¹) were attributed to Fe²⁺ ions in almandine garnet. Runciman³ also feels that the supernova bands are due mainly to Fe, but he suggests that they may mark transitions in Fe²⁺ ions in pyroxene minerals. Many stars have appreciable excess emission at 10 and 20 µm, the origin of which has been attributed to circumstellar silicates^4–7, possibly orthopyroxenes and olivines.